1. Field of the Invention
The present invention relates to an electrode plate for a secondary battery with a nonaqueous electrolyte (referred to "electrode plate" hereinafter), for example, represented by a lithium ion secondary battery and also relates to a process for producing such an electrode plate. In particular, it relates to the process for producing the electrode plate which is capable of economically forming an active material layer and a noncoated portion in accordance with a predetermined pattern on a collector surface, and the electrode plate produced by such a process.
2. Description of the Related Art
In recent years, reduction in size and weight of electronic equipment and communication equipment has rapidly been advanced. This advance has also required reduction in size and weight of batteries used as a driving power source for these equipment. For this request, there has been proposed commercialization of secondary batteries with a nonaqueous electrolyte in which lithium ion secondary batteries having high voltage and high energy density were exemplified as a typical example, in replacement of conventional alkaline batteries.
Regarding an electrode plate which has a great influence on the performance of the secondary batteries with a nonaqueous electrolyte, there has also been proposed reduction in thickness and enlargement in area of the electrode plate, in order to elongate a charge/discharge cycle life and obtain a high energy density. For examples, Japanese Patent Laid-open Publication Nos. 10456/1988 and 285262/1991 disclose positive electrode plates which are produced by the steps of: dispersing or dissolving an active material powder for the positive electrode plate, which is composed of metallic oxides, sulfides, halides or the like, a conductive agent and a binder into a suitable wetting agent (referred to as "solvent" hereinafter) to prepare an active material coating solution in the form of paste; and applying this active material coating solution on a surface of a collector as a substrate made of a metallic foil to form a coating layer (active material layer). In this process, as the binder, for example, there is used fluororesin such as polyvinylidene fluoride or the like or silicone-acrylic copolymer. On the other hand, negative electrode plates has been produced by the steps of: adding such an active material powder for the negative electrode plate as carbon into a solution or dispersion of a binder in a suitable solvent to prepare an active material coating solution in the form of paste; and applying this active material coating solution on a surface of a collector made of a metallic foil.
In the above-mentioned coating type electrode plate, the binder for preparing the active material coating solution requires an electrochemical stability and insolubility against the nonaqueous electrolyte, and solubility in a certain solvent for a coating process. Furthermore, the active material layer (coating layer) of the electrode plate, which is obtained by applying the active material coating solution on the collector of the metallic foil and drying same, requires sufficient flexibility and adhesive property to prevent peeling, chipping, cracking or the like at the assembling process and the charge/discharge process of the battery.
As usual, for the electrode plate, the presence of the coating layer is unfavorable for a certain portion thereof, for example, a portion to which a terminal for introducing an electric current is connected or a portion at which the electrode plate is bent for preparing a battery. For this reason, the electrode plate is usually provided with the active material layer formed in a predetermined pattern and at least one non-coated (or non-coating) portion on which the active material layer is not disposed. However, when a coater of the conventional art is used to form coated portions (i.e., portions provided with the active material layer) and noncoated portions (i.e., boundary portions between the active material layers, portions to which the terminals are to be connected, or the like portions) on the collector surface in a continuous manner at a high speed, it comes necessary that a coater head alternately repeats to be brought into contact with the collector surface and brought apart therefrom.
For example, when a coating speed of the coater goes higher while the collector surface is subjected to the coating process in a continuous manner with a width of the coated portion being set for 60 cm and a width of the noncoated portion being set for 5 cm, a contact time of the coater head to the collector surface is shortened, causing difficulty of a mechanical control of the coater head for applying the coating solution in a certain pattern. Accordingly, it is difficult to form the coated portion and the noncoated portion having accurate patterns respectively in the continuous manner at a high coating speed. On the other hand, a low coating speed prevents the problem of accuracy, but causes a problem of productivity of the electrode plate.